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<main>
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<h1 id="newtype-and-sized"><a class="header" href="#newtype-and-sized">newtype and Sized</a></h1>
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<h2 id="newtype"><a class="header" href="#newtype">Newtype</a></h2>
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<p>The orphan rule tells us that we are allowed to implement a trait on a type as long as either the trait or the type are local to our crate.</p>
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<p>The <strong>newtype pattern</strong> can help us get around this restriction, which involves creating a new type in a <strong>tuple struct</strong>.</p>
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<ol>
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<li>🌟</li>
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</ol>
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<pre><pre class="playground"><code class="language-rust editable edition2021">use std::fmt;
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/* Define the Wrapper type */
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__;
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// Display is an external trait
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impl fmt::Display for Wrapper {
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fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
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write!(f, "[{}]", self.0.join(", "))
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}
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}
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fn main() {
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// Vec is an external type, so you cannot implement Display trait on Vec type
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let w = Wrapper(vec![String::from("hello"), String::from("world")]);
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println!("w = {}", w);
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}</code></pre></pre>
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<ol start="2">
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<li>🌟 Hide the methods of the original type.</li>
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</ol>
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<pre><pre class="playground"><code class="language-rust editable edition2021">/* Make it workd */
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struct Meters(u32);
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fn main() {
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let i: u32 = 2;
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assert_eq!(i.pow(2), 4);
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let n = Meters(i);
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// The `pow` method is defined on `u32` type, we can't directly call it
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assert_eq!(n.pow(2), 4);
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}</code></pre></pre>
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<ol start="3">
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<li>🌟🌟 The <code>newtype</code> idiom gives compile time guarantees that the right type of value is supplied to a program.</li>
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</ol>
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<pre><pre class="playground"><code class="language-rust editable edition2021">/* Make it work */
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struct Years(i64);
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struct Days(i64);
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impl Years {
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pub fn to_days(&self) -> Days {
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Days(self.0 * 365)
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}
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}
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impl Days {
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pub fn to_years(&self) -> Years {
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Years(self.0 / 365)
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}
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}
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// An age verification function that checks age in years, must be given a value of type Years.
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fn old_enough(age: &Years) -> bool {
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age.0 >= 18
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}
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fn main() {
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let age = Years(5);
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let age_days = age.to_days();
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println!("Old enough {}", old_enough(&age));
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println!("Old enough {}", old_enough(&age_days));
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}</code></pre></pre>
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<ol start="4">
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<li>🌟🌟</li>
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</ol>
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<pre><pre class="playground"><code class="language-rust editable edition2021">use std::ops::Add;
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use std::fmt::{self, format};
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struct Meters(u32);
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impl fmt::Display for Meters {
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fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
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write!(f, "There are still {} meters left", self.0)
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}
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}
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impl Add for Meters {
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type Output = Self;
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fn add(self, other: Meters) -> Self {
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Self(self.0 + other.0)
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}
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}
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fn main() {
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let d = calculate_distance(Meters(10), Meters(20));
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assert_eq!(format!("{}",d), "There are still 30 meters left");
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}
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/* Implement calculate_distance */
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fn calculate_distance</code></pre></pre>
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<h2 id="type-alias"><a class="header" href="#type-alias">Type alias</a></h2>
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<p>Type alias is important to improve the readability of our code.</p>
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<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>type Thunk = Box<dyn Fn() + Send + 'static>;
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let f: Thunk = Box::new(|| println!("hi"));
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fn takes_long_type(f: Thunk) {
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// --snip--
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}
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fn returns_long_type() -> Thunk {
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// --snip--
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}
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<span class="boring">}</span></code></pre></pre>
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<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>type Result<T> = std::result::Result<T, std::io::Error>;
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<span class="boring">}</span></code></pre></pre>
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<p>And Unlike newtype, type alias don't create new types, so the following code is valid:</p>
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<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>type Meters = u32;
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let x: u32 = 5;
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let y: Meters = 5;
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println!("x + y = {}", x + y);
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<span class="boring">}</span></code></pre></pre>
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<ol start="5">
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<li>🌟</li>
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</ol>
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<pre><pre class="playground"><code class="language-rust editable edition2021">enum VeryVerboseEnumOfThingsToDoWithNumbers {
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Add,
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Subtract,
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}
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/* Fill in the blank */
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__
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fn main() {
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// We can refer to each variant via its alias, not its long and inconvenient
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// name.
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let x = Operations::Add;
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}</code></pre></pre>
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<ol start="6">
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<li>🌟🌟 There are a few preserved aliases in Rust, one of which can be used in <code>impl</code> blocks.</li>
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</ol>
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<pre><pre class="playground"><code class="language-rust editable edition2021">enum VeryVerboseEnumOfThingsToDoWithNumbers {
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Add,
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Subtract,
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}
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impl VeryVerboseEnumOfThingsToDoWithNumbers {
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fn run(&self, x: i32, y: i32) -> i32 {
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match self {
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__::Add => x + y,
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__::Subtract => x - y,
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}
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}
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}</code></pre></pre>
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<h2 id="dst-and-unsized-type"><a class="header" href="#dst-and-unsized-type">DST and unsized type</a></h2>
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<p>These concepts are complicated, so we are not going to explain here, but you can find them in <a href="https://doc.rust-lang.org/book/ch19-04-advanced-types.html?highlight=DST#dynamically-sized-types-and-the-sized-trait">The Book</a>.</p>
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<ol start="7">
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<li>🌟🌟🌟 Array with dynamic length is a Dynamic Sized Type ( DST ), we can't directly use it</li>
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</ol>
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<pre><pre class="playground"><code class="language-rust editable edition2021">/* Make it work with const generics */
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fn my_function(n: usize) -> [u32; usize] {
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[123; n]
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}
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fn main() {
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let arr = my_function();
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println!("{:?}",arr);
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}</code></pre></pre>
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<ol start="8">
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<li>🌟🌟 Slice is unsized type, but the reference of slice is not.</li>
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</ol>
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<pre><pre class="playground"><code class="language-rust editable edition2021">/* Make it work with slice references */
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fn main() {
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let s: str = "Hello there!";
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let arr: [u8] = [1, 2, 3];
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}</code></pre></pre>
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<ol start="9">
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<li>🌟🌟 Trait is also an unsized type</li>
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</ol>
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<pre><pre class="playground"><code class="language-rust editable edition2021">/* Make it work in two ways */
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use std::fmt::Display;
|
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fn foobar(thing: Display) {}
|
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|
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fn main() {
|
|
}</code></pre></pre>
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